The United States has for the first time called on countries to stop buying Syrian oil and gas as it seeks to build pressure on the country's president Bashar al-Assad to end a brutal crackdown against protesters.

The US secretary of state, Hillary Clinton, hinted that Washington might be making some headway in persuading European nations, China and India to curtail their energy ties with Syria, saying "stay tuned" when asked what progress had been made.

Clinton has said that the best way pressure Syria into stopping its crackdown on protesters calling for the end of the Assad regime was to boycott its oil and gas sector. This, however, would largely fall to other nations because there is little US involvement.

Defiant protest marches unfolded across the country including in the cities of Hama and Deir al-Zor, both of which have been stormed in tank offensives during Ramadan.

Speaking at a news conference with the Norwegian foreign minister, Jonas Gahr Stoere, Clinton stopped short of explicitly calling for Assad's departure.

"We urge those countries still buying Syrian oil and gas, those countries still sending Assad weapons, those countries whose political and economic support give him comfort in his brutality, to get on the right side of history," she said.

US officials who spoke on condition of anonymity said it was the first time Washington had explicitly called for a boycott of Syrian oil since the protests erupted in March after popular uprisings toppled authoritarian leaders in Egypt and Tunisia.

Syria's crude oil production of 380,000 barrels a day generates most of the state's hard currency. According to the US Energy Information Administration, Syrian crude exports go mostly to European nations such as Germany, Italy and France.

"President Assad has lost the legitimacy to lead and it is clear that Syria would be better off without him," she added at the news conference, echoing previous comments by the Obama administration.

Activists say more than 1,700 civilians have been killed in the military crackdown on the protests. Syria blames the violence on armed groups, which it said had killed 500 soldiers and police.